(1) Field of the Invention
The present invention generally relates to constant-current circuits, and more particularly to a constant-current circuit for a light-emitting element for use in, for example, a communication system.
(2) Description of the Prior Art
An optical communication device, such as an optical repeater, has a light-emitting element and a constant-current circuit for supplying a constant current to the light-emitting element.
FIG. 1 is a block diagram of a conventional constant-current circuit for a light-emitting element, such as a laser diode (see Japanese Laid-Open Patent Application No. 58-171140). The constant-current circuit shown in FIG. 1, connected to a laser diode LD, is composed of an input signal break detection/control circuit 11, a laser diode driving circuit 12, a Zener diode 13, an npn transistor Q, and resistors R1 and R2. The control circuit 11, which includes a peak-to-peak detection circuit, converts a pulse-shaped input signal (which corresponds to an optical input signal applied to the optical repeater) from a terminal 10 into a D.C. current signal, and generates a bias signal from the D.C. current signal. The bias signal is applied to the laser diode driving circuit 12. While the control circuit 11 is not receiving the input signal, the control circuit 11 generates a base current, which is applied to the base of the transistor Q via the resistor R1.
The laser diode LD is turned ON and OFF by the laser diode driving circuit 12 in response to the input signal. An optical output signal is emitted from the laser diode LD when the optical input signal is input to the optical repeater. During this state, the transistor Q is maintained in the OFF state since the control circuit 11 does not generate the base current of the transistor Q. When the optical input signal is not received by the optical repeater, the laser diode driving circuit 12 does not receive the input signal and the bias signal, and thus the laser diode LD does not emit any optical output signal. During this time, the control circuit 11 supplies the base current to the base of the transistor Q via the resistor R1. Hence, the transistor Q is turned ON, and a current from a line L1 passes to a line L2 through a current bypass circuit composed of the transistor Q and the resistor R2.
When the optical input signal is not applied to the optical repeater, a decreased amount of current is consumed in the laser diode driving circuit 12, as compared with the amount of current obtained when the optical repeater receives the optical input signal. For this reason, the control circuit 11 generates the base current so that an increased amount of current passes through the bypass circuit composed of the transistor Q and the resistor R2 in order to compensate for the decrease in the current consumed in the laser diode driving circuit 12. In this manner, it becomes possible to pass a fixed amount of current through the Zener diode 13. The above operation can be described as follows: EQU I.sub.LD =I.sub.LD '+I (1)
where I.sub.LD is the current consumed in the laser diode driving circuit 12 when the optical input signal is received, I.sub.LD ' is the current consumed in the laser diode driving circuit 12 when the optical input signal is not received, and I is the current passing through the transistor Q. When the equation (1) is satisfied, the current passing from the line L1 to the line L2 does not change irrespective of whether or not the optical input signal is received.
However, it is impossible for the conventional constant-current circuit to compensate for a change in the bias current arising from a change in ambient temperature and/or deterioration of the laser diode.